This story has been updated.

As India, the world’s second-most populous country, reels from an intense drought, the World Bank has released a new report finding that perhaps the most severe impact of a changing climate could be the effect on water supplies.

The most startling finding? The report suggests that by 2050, an inadequate supply of water could knock down economic growth in some parts of the world a figure as high as 6 percent of GDP, “sending them into sustained negative growth.” Regions facing this risk — which can at least partly be averted by better water management, the document notes — include not only much of Africa but also India, China and the Middle East.

 “When we look at any of the major impacts of climate change, they one way or another come through water,” said Richard Damania, a lead economist at the bank and the lead author of the report, on a call with reporters Tuesday. “So it will be no exaggeration to claim that climate change is really in fact about hydrological change.”

Climate change hits water supplies in multiple ways. Warm temperatures can cause more evaporation of water from landscapes, while changes in precipitation can lead to both more intense individual downpours but also swings into drought conditions. The threat from all this is not just to what people drink but what they eat: The human activity that consumes the most water is agriculture.

And then, there’s sea-level rise: It can push into coastal aquifers, as is happening today in the state of Florida, and thus threaten to make them more saline and less usable for human needs. So it isn’t only surface waters that may be depleted by climate swings, but also groundwater.

The World Bank report says that 1.6 billion people on Earth already live in nations that are subject to water scarcity. Depending on the precise definition of the concept, other research has put that number even higher, finding that 4 billion live in regions that face conditions of “severe” water scarcity during at least some part of the year. Using its own definition, the World Bank fears the number of people living with potential water threats will double over the next two decades.

The problem will be exacerbated by greater populations overall, and more demand for water due to increased needs in the electricity generation and agricultural sectors. But the impacts, the study found, will also be very uneven, with little projected economic harm to North America or Europe from water supply changes.

“Growing populations, rising incomes, and expanding cities will converge upon a world where the demand for water rises exponentially, while supply becomes more erratic and uncertain,” the report says.

It’s not the case that the world as a whole will have inadequate fresh water — it’s that some places will be fine but others won’t have enough, and there’s not much means of mass water redistribution over long distances. Rather, grapping with these looming water scarcity problems has to occur in specific regions, where it will be important to address water waste, misalloaction, and efficiency in water use — in other words, using the same amount of water for more diverse purposes or needs.

That’s a crucial task, the report finds, due to staggering projected increases in fresh water demand. The report finds that in the next 30 years, “the global food system will require between 40 to 50 percent more water; municipal and industrial water demand will increase by 50 to 70 percent; the energy sector will see water demand increase by 85 percent; and the environment, already the residual claimant, may receive even less.”

And when water shortages happen, the poor will inevitably be hit the hardest — when it comes to both food and drinking water, because they may not be able to purchase supplies from elsewhere to get them through hard time.